The present invention relates to a process and to an apparatus for the gas-phase polymerization of olefin(s) in a fluidized-bed reactor.
It is known to carry out a process for the gas-phase polymerization of olefin(s) in a fluidized-bed reactor fed with olefin(s), with catalyst and optionally with cocatalyst, within which reactor solid particles of polymer in the course of formation, of polymer formed and of catalyst are maintained in the fluidized state by the action of an ascending gas stream, comprising the olefin or olefins, starting from the base of the reactor and rising up to its top part.
The fluidized-bed reactor generally comprises, in its lower part, a fluidization grid, ensuring uniform distribution of the ascending gas stream through the bed, in its central part, a vertical cylindrical barrel comprising the fluidized bed and, in its upper part, a so-called disengagement chamber, in which the ascending gas stream is slowed down because the transverse cross-section of this chamber is wider than that of the cylindrical barrel, so that a substantial portion of the solid particles entrained out of the bed is returned to the bed.
Given that the polymerization reaction is generally exothermic and that, moreover, only a portion of the olefin or olefins reacts when passing through the reactor, the process comprises a recycling loop in which the ascending gas stream withdrawn at the top part of the reactor circulates. The gas stream is moved in the recycling loop by a compressor, cooled by at least one heat exchanger and reinjected at the bottom part of the reactor with a velocity sufficient to maintain the solid particles in the fluidized state in the bed.
The recycling loop usually comprises at least one pipe for conveying the gas stream withdrawn at the top of the reactor, optionally at least one gas/solid separator capable of separating, from the gas stream, a substantial portion of the solid particles entrained with it out of the reactor and in retaining them out of the gas stream, at least one heat exchanger for cooling the gas stream, at least one compressor for circulating the gas stream in the loop and at least one pipe for conveying the gas stream in order to introduce it at the bottom part of the reactor, in particular under the fluidization grid.
Despite the presence of the disengagement chamber at the upper part of the reactor and optionally of a gas/solid separator positioned on the recycling loop, the gas stream circulating in the loop can be laden with fine solid particles. These particles can be deposited and can pile up in a portion of the loop, and can react with the olefins and can form a polymer mass capable of partially blocking the loop, for example a portion of the heat exchanger at least.
In order to overcome this problem, Patent FR-A-2,634,212 provides for making available a heat exchanger in the loop and for injecting a liquid hydrocarbon upstream of the exchanger, in order to wash and clean the interior of the exchanger.
Furthermore, International Patent Application WO9820046 provides, with the same aim, for the introduction, at one or more points in the loop, of an agent which prevents the deposition of polymer particles and which is composed of an alcohol, of an alkyl ether, of ammonia, of an inorganic acid ester, of a compound from Group IV of the Periodic Classification of the Elements, of a compound comprising sulphur, of alkylamine or of a mixture of these.
However, one or other of the methods provided consists in adding a component to the gas stream circulating in the recycling loop, which component modifies the composition of the gas stream and can thus disrupt the polymerization reaction in the fluidized bed.
The U.S. Pat. No. 4,588,790 discloses a continuous gas fluidised bed process for the production of polymer, continuously passing a gaseous stream through a fluidised bed in a reactor zone, withdrawing from this reactor zone polymer product and a stream comprising unreacted gases and solid particles, cooling said stream to condense a portion thereof and form a liquid containing mixture wherein the weight ratio of liquid to solid particles is not less than two to one. This method is used to avoid the formation of wet agglomerate solid particles which accumulate in regions of low velocity in the recycle lines and heat exchanger.
This document does not describe the essential characteristics of the invention and does not address the problem of deposition, accumulation and/or piling up of particles in the recycling loop in general terms, whatever the conditions of polymerization and, particularly when it is not desired or possible to condense part of the recycled stream.
Numerous efforts have been made for a long time to find a solution to this problem. In particular, it was found that this problem became even more acute when an additional heat exchanger was installed in the recycling loop of a fluidized-bed reactor for the purpose of increasing the heat exchange capacity and consequently the polymer production of the reactor. In addition, the only way of adding this exchanger to the loop was to position it horizontally, due to the lack of space in the plant. The additional heat exchanger was of multitubular type, successively comprising an inlet chamber, a bundle of tubes with a horizontal longitudinal axis which is encased in a horizontal cylindrical shell, and an outlet chamber, the inlet and outlet chambers each being essentially composed of the casing of a right truncated cone (also known as a right circular truncated cone) with a horizontal axis of symmetry identical to that of the cylindrical shell. Very rapid fouling of the bottom part of the exchanger then took place. The fouling was formed by fine solid polymer particles entrained by the gas stream circulating in the recycling loop. These particles were deposited on the internal walls of the heat exchanger and formed a solid polymer plug which gradually expanded as the polymerization reaction continued. This resulted in a gradual loss of the heat exchange capacity and an increase in the pressure drop in the exchanger, so that it became necessary to increase the power of the compressor in order to maintain sufficient circulation of the gas stream in the recycling loop and in the fluidized bed.
The European Patent Application 0 042 892 discloses an apparatus for recovery of condensable vapours in wasted gas to be flared comprising a gas-to-liquid heat exchanger and a mean for separating the condensed vapour from the remaining flare gas.
The German Patent Application 1 005 993 discloses a heat exchanger equipped with a bundle of tubes with a mechanical configuration allowing an easy dismantling.
Both documents disclose neither the use of heat exchangers in a gas phase process for polymerizing olefins, nor the problem of deposition, accumulation and/or piling up of particles in the heat exchanger.
The object of the present invention is to provide a polymerization process and a polymerization apparatus equipped with means which limit or prevent the deposition, the accumulation and/or the build up of particles in the recycling loop and in particular in the heat exchanger, with the advantage of not introducing into the loop any additional component capable of disrupting the polymerization reaction.